<!DOCTYPE html>
<html lang="zh-cn">
    <head>
        <meta charset="UTF-8"/>
        <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
        <title>Document</title>
        <style>
            *{
                padding: 0;
                margin: 0;
            }
            #circle{
                position: absolute;
                left: 0;
                top: 0;
                width: 200px;
                height: 200px;
                border:1px solid #000;
                box-sizing: border-box;
                border-radius:50%;
            }
            .dot{
                position: absolute;
                left: 200px;
                top: 100px;
                height: 4px;
                width: 4px;
                margin-left: -2px;
                margin-top: -2px;
                background-color: red;
            }
            #circleCenter{
                position: absolute;
                left: 100px;
                top: 100px;
                width: 6px;
                height: 6px;
                margin-left: -3px;
                margin-top: -3px;
                background-color: #f00;
            }
        </style>
    </head>
    <body>
        <div id="circle"></div>
        <div id="circleCenter"></div>
        <div id="dot" class="dot"></div>
        <script src="http://cdn.bootcss.com/jquery/1.10.2/jquery.min.js"></script>
        <script>
            var r = 100;   //圆形半径

            var hudu = (2*Math.PI / 360) *(-30);   //  360/8=45,即45度(这个随个人设置)

             var X = 100 + Math.sin(hudu) * r;    //  95 是圆形中心的坐标X   即定位left 的值

            var Y = 100 + Math.cos(hudu) * r;    //  95 是圆形中心的坐标Y   即定位top 的值

            $('#dot').css({

                 left:X+"px",

                 top:Y+"px",

            }) 
        </script>
    </body>
</html>